The present invention relates to the control of hydraulic fluid for selectively effecting the engagement of speed and direction-friction devices of a power shift transmission. More specifically, the present invention relates to the control of the rate of modulation of the fluid for effecting engagement of the forward and reverse direction-friction devices.
The desirability of effecting soft shifts in a transmission, in order to avoid wear and tear in the components thereof and also to avoid operator discomfort, is well-known. It is further a well-known expedient to accomplish the above desirable results by modulating the flow of fluid for engaging the friction devices of the transmission. The modulation in the controls of the prior art devices is normally effected by a modulating valve having a spring whose effective length is changed by stroking a modulating piston engaged with one end of the spring each time the transmission is shifted. An example of such a modulating valve can be seen in U.S. Pat. No. 3,487,851, issued to Golan et al on Jan. 6, 1970, wherein there is shown a modulating piston controlled by a source of control fluid which, during the initial movement of the piston, is routed to the piston by a path passing through a pair of restricted passages and then, as the piston moves further, only through one of the restricted passages. Thus, a dual rate of modulation for each shift is obtained.
Another example of a transmission control which employs a pressure-regulating valve, having a piston which is stroked during each shift, is disclosed in U.S. Pat. No. 3,799,308, issued to Erisman on Mar. 26, 1974. The pressure-regulating valve, shown in the Erisman patent, includes a piston through which fluid is routed by means of a series of feedback lines which are in fluid communication with the actuators for each of the friction devices. These feedback lines respectively include various orifices which operate to cause the regulating valve piston to stroke at different rates for each of the different speeds and/or direction combinations.
The present invention is somewhat akin to the control system disclosed in the Erisman patent in that it deals with the idea of effecting different rates of pressure modulation for each of the selected speeds.